Hard wood, i.e. wood of relatively high density, is known for its attractive properties such as a high mechanical strength and a low tendency to absorb moisture. Hence it is a valuable material which is widely applied, both for indoor and outdoor uses. However, the sources of its supply are limited, as hard wood producing trees are generally slow growing and need many years to reach the proper dimensions for the intended outlets. Also, the utilization of hard wood producing trees is subject to limitations for environmental reasons.
Light wood producing trees, on the other hand, are usually fast growing and accordingly could easily provide the amounts of wood commercially needed. However, the relatively poor mechanical properties of light wood and its high capacity of absorbing moisture, making it open to attack by funghi and various plant diseases, prevent the direct use of this type of wood for most of the outlets of present interest.
Therefore, for many years there has been an incentive to upgrade the low-density light wood, so that this wood could likewise be used for those applications, for which so far mainly hard wood had been considered.
The preparation of cellulosic fibrous aggregates and in particular that of light wood aggregates is known in the art. Various attempts have been made to convert relatively small sections of trees into sections of larger size and to improve the properties of the aggregates obtainable from these sections.
In EP-A-161766 a process is disclosed for converting lignocellulosic materials into reconstituted products. The process comprises treating the lignocellulosic material in divided form with steam to heat the material to a temperature high enough to release hemicellulose but not exceeding the temperature of carbonization, for a time sufficient to decompose and hydrolyse hemicellulose into free sugars, sugar polymers, dehydrated carbohydrates, furfural product and other decomposition products; forming the treated lignocellulosic material into a mat and pressing the mat at a temperature not exceeding the temperature at which the mat would char, at a pressure and for a time sufficient to transform and thermoset the free sugars, sugar polymers, dehydrated carbohydrates, furfural products and other decomposition products in the lignocellulose material into a polymeric substance which adhesively bonds together the lignocellulosic material to yield the reconstituted composite product. However, the disclosure, in particular the specific examples, is limited to the treatment of divided starting material, that is material in which elongate cellulosic fibres are not present. Since the inherent strength of the products results from the presence of a network of elongate cellulosic fibres, the properties of the products obtained in the known process are not satisfactory.
A more attractive process for preparing a moisture resistant cellulosic fibrous aggregate from a cellulosic fibrous material is disclosed in EP-A-373726. The process comprises a softening stage comprising exposing a section of cellulosic fibrous material to the action of an aqueous softening agent at a temperature in the range of from 150.degree. C. to 220.degree. C. and at a pressure of at least the equilibrium vapour pressure of the softening agent at the operating temperature, thereby at least partially disproportionating and hydrolising the hemicellulose and lignin present in the cellulosic fibrous material, and a curing stage comprising drying the product of the softening stage at a temperature in the range of from 100.degree. C. to 220.degree. C. to yield a cross-linked cellulosic matrix.
The term "section" when used in relation to the starting material for the process used to form the aggregate is a reference to a portion of cellulosic fibrous material for example at least 20 cm long and having a cross-section with a dimension for example of at least 5 mm. Such pieces should be distinguished from the pulp, powder, shavings or chips of other prior art processes.
In the said process, disclosed in EP-A-373726, the softening agent may be present as water or as steam. It is mentioned that a preferred method of exposing the starting material to the softening agent consists in allowing steam to condense on the surface of the starting material.
Although the use of steam is advantageous in that it is a simple and direct method of supplying heat, it appears that the use of steam in other aspects is less advantageous, e.g. as regards the cooling of the product obtained in the softening stage.
As regards the use of water as a softening agent, it was initially believed that in large scale embodiments of the process the heat required for the softening stage could not be supplied in a commercially viable manner.